This paper report results of the five years of an on going research project on nutrient balances in a composite swidden agroecosystem. The study is being carried out in Tat hamlet, a small settlement of Da Bac Tay ethnic minority people in Hoa Binh province in Vietnam’s Northern Mountain Region. Nutrient flows from the upland to the paddy field were directly proportional to the amount of rainfall and prevailing soil surface/cropping conditions. Nutrients flows from the upland to paddy fields in this experiment increased from 97 to 227 kg total N between 2000 and 2001 and decreased sharply thereafter, to 78 kg N in 2002 and to 50 kg in 2004. P and K outflows from the watershed showed similar trends.
On average, the inflows of available nutrients to the upper 3 paddy fields were equivalent to available nutrient additions of as much as 453; 58 and 1455 kg/ha for N, P and K respectively. Even when considering the whole paddy area, potential inputs could on average account for up to 151; 19 and 485 kg available N, P and K respectively. Potential total nutrient contributions were even larger. However, in reality, farmers would not divert all of the discharge into the paddy fields, e.g. paddies are drained during rice harvest of summer rice.
The net nutrient flow balance varied considerably between years. In 2000, N flow balances were positive for both total and available N. However, available N was reduced to -0.7 kg in 2001 and even to -21.0 kg in 2002. Net flow P balances varied least. Potassium net in-outflow balances showed as large of a yearly variation as N. Potassium net flow balances, although mostly negative in the first years, became positive in the last year.
